1. Field of the Invention
This invention relates to the surface detection of low level bioelectric signals and, more particularly, to the surface detection of ventricular late potentials.
2. Description of the Prior Art
Sudden death following apparent recovery from uncomplicated acute myocardial infarction is not an uncommon event in modern medicine. It is usually ascribed to the unforeseen development of malignant ventricular arrhythmias that do not spontaneously terminate. Ventricular arrhythmias can lead to ventricular tachycardia, a condition in which the heart beats rapidly, pumping only a minimal amount of blood. During attacks of ventricular tachycardia, the patient may collapse due to an inadequate blood supply. Ventricular tachycardia can lead to ventricular fibrillation, a situation in which the heart simply quivers, pumping no blood at all. The patient will die unless immediate medical treatment reestablishes synchronous beating of the heart.
Post myocardial infarction patients who are at risk for sudden death from ventricular arrhythmias may have no indication until a life-threatening event occurs. Those who experience prolonged episodes of ventricular tachycardia have a high mortality rate. Survivors may undergo special invasive electrophysiological testing. Testing involves attempts to induce a ventricular arrhythmia to assess possible susceptibility to future spontaneous development of ventricular arrhythmias. During testing, if sustained, ventricular tachycardia is induced, implying susceptibility, treatment with antiarrhythmics may be initiated followed by further testing for inducibility of arrythmias. Such testing/treatment programs may involve extended hospitalization and trauma.
Within the last twelve years, studies have disclosed that low amplitude high frequency electrical signals called "ventricular late potentials" are often present in the electrocardiograms (ECGs) of patients who, after myocardial infarction, have episodes of potentially dangerous ventricular arrhythmias. Specifically, these ventricular late potentials often follow the terminal portion of the QRS complex or occur during the ST segment, T-wave or other diastolic portions of the ECG. While the precise origin of these waveforms is unknown, it is believed that these ventricular late potentials are generated by small islands of muscle cells located within cardiac scar tissue. These ventricular late potentials can initiate ventricular arrhythmias.
Because ventricular late potentials are very small amplitude electrical signals, they are difficult to observe in standard ECGs acquired with electrodes placed on the patient's thorax. The ventricular late potentials are obscured at the surface of the thorax by the electrical interference or "noise" from intervening nerve and muscle tissue and environmental noise, particularly 60 Hz and its harmonics.
Several attempts have been made in the prior art to measure the analog ECG waveform, convert the waveform to a digital signal and then digitally filter out the noise component in order to isolate, enhance and identify whether ventricular late potentials are present in ECGs measured on the surface of the body.
Simson, "Use of Signals in the Terminal QRS Complex to Identify Patients with Ventricular Tachycardia After Myocardial Infarction", Circulation 64:2, 1981, first demonstrated the existence of small, high frequency electrocardiographic potentials in ECGs measured on the surface of the body of post myocardial infarction patients. He digitized and averaged bipolar leads orthogonally oriented along X, Y and Z axes of a cartesian coordinate system of a patient's thorax. Each lead average was digitally filtered. Simson's filter does not reduce either myoelectric artifacts or environmental noise (60 Hz and its harmonics), which are major components that can obscure ventricular late potentials in the ECG.
Nonetheless, Simson reported two useful measures for identifying the presence of ventricular late potentials in the ECG signal: 1) root-mean-squared (RMS) amplitude of the last 40 milliseconds of the QRS complex (V.sub.rms40); and 2) duration of the QRS complex. Another measure that has come into use is the duration of the interval from offset of the QRS complex back to that point in the QRS complex where its amplitude first exceeds 40 microvolts, an experimentally determined threshold; this is usually termed the low amplitude signal duration (LASD). Ventricular late potentials are low amplitude signals in the tail of the QRS complex. If the RMS amplitude (V.sub.rms40) of the tail (or last 40 milliseconds) of the QRS complex is found to exceed an experimentally established value, it is assumed that the signal is simply a portion of the tail of the QRS complex itself, not a ventricular late potential. However, if the RMS amplitude falls below the experimentally established value, the signal may be a ventricular late potential.
Measurement of the overall duration of the QRS complex assumes that its duration will increase substantially from the typical average of 100 milliseconds in the presence of a ventricular late potential because the ventricular late potential occurs late in the QRS complex, thus lengthening the overall time or duration of the QRS complex waveform. For the LASD measurement, it is assumed that where there is no ventricular late potential, the interval of time from the end of the QRS complex "backwards" in time into the QRS complex where the QRS complex first exceeds 40 millivolts will be 40 milliseconds or less. If a ventricular late potential is present, this interval of time will increase due to the occurrence of ventricular late potential. Abnormal values for these three measurements indicating the existence of ventricular late potentials are: 1) V.sub.rms40 equal to or less than 25 microvolts; 2) QRS duration greater than or equal to 120 milliseconds; and 3) LASD equal to or greater than 40 milliseconds.
Cain, et al., "Quantification of Differences in Frequency Content of Signal Averaged Electrocardiograms in Patients with Compared to Patients Without Sustained Ventricular Tachycardia", American Journal of Cardiology, 55: 1500, 1985, attempted to differentiate normal bipolar lead signals from those containing late potentials on the basis of frequency content. El-Sherif, et al., "Appraisal of a Low Noise Electrocardiogram", Journal of the American College of Cardiology, 1(2):456, 1983, used low-noise techniques and spatial averaging of 16 simultaneously recorded bipolar signals to identify low amplitude, late diastolic potential s beat-to-beat in the ST segment of post myocardial infarction patients with a propensity for development of ventricular arrhythmias. Hombach, et al. , "Noninvasive Beat-by-beat Registration of Ventricular Late Potentials Using High Resolution Electrocardiography", International Journal of Cardiology, 6:167, 1984, attempted beat-to-beat registration of ventricular late potentials by using spatial averaging of four signals in conjunction with specially designed suction electrodes and low noise preamplifiers. To reduce environmental noise, Hombach, et al. performed the tests inside a Faraday cage, limiting the clinical accessibility to this test.
The use of a technique known as "time-sequenced adaptive filtering" (TSAF) for removing noise from a measured signal has been investigated. Ferrera, "The Time-Sequenced Adaptive Filter", Ph.D. thesis, Stanford University, 1978, first reported on a technique of using TSAF as a refinement of the least-mean-squared-enhancer developed by Widrow, "Stationary and Non-stationary Learning Characteristics of the LMS Adaptive Filter", Proceedings of the IEEE, 64:1151, 1976.
Problems associated with the use of the Ferrera algorithm in detecting bioelectric signals (specifically signals generated by the heart's His-Purkinje system) have been investigated. M. T. Juran, "Surface Recordings of His-Purkinje Activity Using Adaptive Filtering", Masters Thesis, Carnegie-Mellon University, 1984, investigated the effects of correlated noise in the input signals to the adaptive filter and devised an adjustment factor to minimize the effects of correlated noise, depending on the degree of correlation.
U.S. Pat. No. 4,751,931, issued Jun. 21, 1988, to Briller, et al., for a "Method and Apparatus for Determining His-Purkinje Activity", developed an improved method and apparatus utilizing TSAF for facilitating observation of His signals in surface ECG signals. Briller, et al. preserves the amplitude and high frequency characteristics of the sharp His signal in a real time data processing apparatus. The device filters out background noise to enhance the His signal in a very short time, typically ten to eleven heart beats, and often preserves beat-to-beat changes. However, Briller, et al. is not suitable for the measurement of ventricular late potentials because it does not provide an accurate estimate of the signal amplitude which is a necessary factor in identifying ventricular late potentials. Further, it would be preferred to have a lower remaining noise after processing in order to identify the weak ventricular late potential signal.
Certain commercially available devices are available for measuring ventricular late potentials. Examples include Predictor SAECG.RTM., available from Corazonix Corporation and the ART 1200 EPX.TM., available from Arrhythmia Research Technology, Inc., of Oklahoma City, Okla. However, these commercial devices are based on Simson's method, typically requiring 200-1000 cardiac cycles to perform analysis and as much as fifteen minutes to acquire enough cycles to identify ventricular late potentials.